1. Field of the Invention
The present invention relates to a thermal transfer type printing device which uses a printing mechanism such as an electrically energized thermal transfer print head and is particularly suitable for printing on a recording medium for use in an overhead projector (OHP). Other printing mechanisms can be used instead, using techniques such as other thermal processes, ink jet printing, electrophotographic printing, etc.
2. Description of the Prior Art
Thermal transfer printing has been used to print on a transparent or semitransparent recording medium for use in a projector such as an OHP (overhead projector). Thermal transfer type printing has been used for this purpose. There are two kinds of OHP's: a reflection type OHP and a transmission type OHP. In the past, OHP sheets have been printed using the same printing equipment for both reflection type and transfer type OHP's. However, when projecting the same OHP sheet on a screen, the screen image from a reflection OHP has higher density than from a transmission type OHP. The fact that the two types of OHP's project the same OHP sheet as a screen image of perceptibly different densities is believed to be a shortcoming of conventional OHP's.
Furthermore, when projecting an OHP image onto a screen, the distance between the OHP and the screen affects the density of the screen image. The change in perceived density with OHP-to-screen distance is even clearer in the case of a full color image.
Thus, when OHP sheets prepared by the same printing process are used in different types of OHP's and/or in OHP's spaced by different distances from the screen or screens, the visual effect can be undesirable due to the differently perceived densities of the screen images.